Lightning-induced remanent magnetic anomalies in low-altitude aeromagnetic data

A low altitude helicopter magnetic survey for unexploded ordnance in New Mexico revealed several magnetic anomalies that were most likely induced by lightning strikes. Lightning-strike magnetic anomalies are not necessarily rare, but may be spaced so widely as to make their detection unlikely in a ground survey. Detailed examples are not often reported because ground geophysical surveys may not cover enough area to detect one, and traditional airborne surveys, which do cover large areas, are carried out at an altitude and line spacing which does not appropriately define the unusual shape of the lightning strike anomaly. However, very low-level (1-3 m altitude) airborne magnetic surveys have data densities similar to ground geophysical surveys, yet cover much larger areas. Lightning anomalies appear in magnetic data as radial arms emanating from a central strike point. Each arm has distinct positive and negative lobes. Anomaly amplitudes in the New Mexico survey area ranged from roughly -30 to +30 nT/m, and the lightning-strike anomaly density was about one per 20 hectares. This is a conservative estimate, as only the most obvious anomalies were counted. The character of the lightning-induced magnetic anomalies changes with an increase in survey altitude or with wider line spacing, making them less distinct from other types of magnetic anomalies.